plumbing/object: don't assign err from ForEach loop

Since we don't check the value anyway, as it can't possibly be anything but nil.

Signed-off-by: Christian Muehlhaeuser <muesli@gmail.com>
(cherry picked from commit 293e70fbceb5c81f33a96d5705f10785c6063db9)
diff --git a/plumbing/object/merge_base.go b/plumbing/object/merge_base.go
index 689e421..6f2568d 100644
--- a/plumbing/object/merge_base.go
+++ b/plumbing/object/merge_base.go
@@ -32,7 +32,7 @@
 	var res []*Commit
 	inNewerHistory := isInIndexCommitFilter(newerHistory)
 	resIter := NewFilterCommitIter(older, &inNewerHistory, &inNewerHistory)
-	err = resIter.ForEach(func(commit *Commit) error {
+	_ = resIter.ForEach(func(commit *Commit) error {
 		res = append(res, commit)
 		return nil
 	})